Surface acoustic wave (SAW) devices are a class of microelectromechanical systems (MEMS) which rely on the modulation of surface acoustic waves to sense a physical phenomenon. The sensor transduces an input electrical signal into a mechanical wave which, unlike an electrical signal, can be easily influenced by physical phenomena. The device then transduces this wave back into an electrical signal. Changes in amplitude, phase, frequency, or time-delay between the input and output electrical signals can be used to measure the presence of the desired phenomenon.
SAW technology takes advantage of the piezoelectric effect in its operation. A basic SAW device consists of a piezoelectric substrate and one or more interdigital transducers (IDTs). The piezoelectric substrate is formed of a piezoelectric material, such as quartz, lithium tantalite (LiTaO3) or lithium niobate (LiNbO3). An IDT is made of two metallic, comb-like structures arranged in an interdigital fashion on the surface of the piezoelectric substrate.
Saw devices typically have two IDTs, i.e., an input IDT and an output IDT. Surface acoustic waves are generated by applying an alternating voltage across the input IDT. The input IDT is fed a sinusoidal electrical input signal which creates alternating polarity between the fingers of the interdigitated transducer. This creates alternating regions of tensile and compressive strain between fingers of the electrode by the piezoelectric effect, producing a mechanical wave at the surface known as a surface acoustic wave.
The wave is received by the output IDT which converts the mechanical energy from the wave back into an electric field using the piezoelectric effect. Any changes that were made to the mechanical wave will be reflected in the output electric signal because the characteristics of the surface acoustic wave will be modified by changes in the surface properties of the piezoelectric substrate.
SAW devices have a resonance frequency which is a function of the spacing between the input and output IDTs and the acoustic wave velocity of the material. When a surface acoustic wave is generated at or near the resonance frequency of the piezoelectric material, the insertion loss of the SAW device can be minimized. However, this also means that the frequency response of the device is established during manufacture. The ability to adjust, or “tune”, the resonance frequency of SAW devices after manufacture is limited. As a consequence, the utilization of SAW devices is typically restricted to applications where the frequency response is fixed.